Substrate proteins, destined for elimination, are initially attached to polymers of the highly conserved ubiquitin (Ub) protein. This covalent modification of the substrate targets the conjugated protein to a multicatalytic protease complex, the 26S proteasome. The Ub attachment site in substrate proteins is commonly a Lys side chain. A well-defined series of enzymes orchestrates the attachment of mono- and polyubiquitin to proteins (see figure). Ub is first activated in an ATP-consuming reaction by an E1 Ub-activating enzyme, to which it becomes attached by a high-energy thioester bond. Subsequently, the activated Ub is transferred to the active site Cys of a second protein, an E2 ubiquitin-conjugating enzyme. With the aid of a third enzyme, called E3 or ubiquitin-protein ligase, E2 catalyses the transfer of (poly)ubiquitin onto the protein that is destined for degradation.

E3 is the most important enzyme in determining the specificity of substrate ubiquitylation. There are two major classes of mechanistically distinct E3 enzymes, characterized by the RING (or RING-like) and HECT domains. Both types of E3 enzymes are alike in their ability to establish selective substrate binding. The RING finger uses Cys and His residues to coordinate a pair of zinc ions in a characteristic arrangement (not shown). A smaller set of E3 enzymes contain a domain called the U box, which is a degenerate version of the RING-finger that achieves the same general fold without coordinating any metal ions. RING and RING-like E3 enzymes bind to both the E2 enzyme and the substrate, and catalyse the transfer of Ub directly from the E2 enzyme to the substrate. Unlike RING and U-box E3 enzymes, the HECT E3 enzymes have a more direct catalytic role in substrate ubiquitylation. The activated Ub of the Ub–E2 enzyme thioester is transferred to a conserved Cys residue in the HECT domain of the E3 before finally being transferred to a substrate.

Ubiquitylationis reversed by de-ubiquitylating enzymes (DUBs) that remove ubiquitin from proteins and disassemble polyubiquitin chains. DUBs provide additional regulatory control before protein degradation, and they are also fundamental for maintaining a sufficient pool of free ubiquitin molecules in the cell.

مهمترین سندرم هایی که در نتیجه اختلال در عملکرد آنزیم E3 ایجاد میشوند:

1-Angelman syndrome

Angelman syndrome(/ˈeɪndʒəlmən/; abbreviatedAS) is a neuro-genetic disordercharacterized by severe intellectual anddevelopmental disability, sleep disturbance,seizures, jerky movements (especially hand-flapping), frequent laughter or smiling, and usually a happy demeanor.

AS is a classic example ofgenomic imprintingin that it is caused by deletion or inactivation of genes on the maternally inheritedchromosome 15while the paternal copy, which may be of normal sequence, is imprinted and therefore silenced. The sister syndrome,Prader-Willi syndrome, is caused by a similar loss of paternally inherited genes and maternal imprinting. AS is named after a Britishpediatrician, Dr.Harry Angelman, who first described thesyndromein 1965. An older, alternative term for AS, "happy puppet syndrome", is generally consideredpejorativeand stigmatizing so it is no longer the accepted term, though it is sometimes still used as an informal term of diagnosis. People with AS are sometimes known as "angels", both because of the syndrome's name and because of their youthful, happy appearance.

Signs and symptoms

The following text lists signs and symptoms of Angelman syndrome and their relative frequency in affected individuals.

Consistent (100%)

  • Developmental delay, functionally severe
  • Speech impairment, no or minimal use of words; receptive and non-verbal communication skills higher than verbal ones
  • Movement or balance disorder, usuallyataxiaof gait and/or tremulous movement of limbs
  • Behavioral uniqueness: any combination of frequent laughter/smiling; apparent happy demeanor; easily excitable personality, often with hand flapping movements; hypermotoric behavior; short attention span

Frequent (more than 80%)

  • Delayed, disproportionate growth in head circumference, usually resulting inmicrocephaly(absolute or relative) by age 2
  • Seizures, onset usually < 3 years of age
  • Abnormal EEG, characteristic pattern with large amplitude slow-spike waves

Associated (20–80%)

  • Strabismus
  • Hypopigmented skin and eyes
  • Tongue thrusting; suck/swallowing disorders
  • Hyperactive tendon reflexes
  • Feeding problems during infancy
  • Uplifted, flexed arms during walking
  • Prominentmandible
  • Increased sensitivity to heat
  • Wide mouth, wide-spaced teeth
  • Sleep disturbance
  • Frequent drooling, protruding tongue
  • Attraction to/fascination with water
  • Excessive chewing/mouthing behaviors
  • Flat back of head
  • Smooth palms

Pathophysiology

Angelman syndrome is caused by the loss of the normal maternal contribution to a region of chromosome 15, most commonly by deletion of a segment of that chromosome. Other causes includeuniparentaldisomy,translocation, or single gene mutation in that region. A healthy person receives two copies of chromosome 15, one from the mother, the other from the father. However, in the region of the chromosome that is critical for Angelman syndrome, the maternal and paternal contribution express certain genes very differently. This is due to sex-specific epigeneticimprinting; the biochemical mechanism isDNA methylation. In a normal individual, the maternalalleleis expressed and the paternal allele is specifically silenced in the developing brain. If the maternal contribution is lost or mutated, the result is Angelman syndrome. (When the paternal contribution is lost, by similar mechanisms, the result isPrader-Willi syndrome.) It should be noted that the methylation test that is performed for Angelman syndrome (a defect in UBE3A) is actually looking for methylation on the gene's neighborSNRPN(which is silenced by methylation on the maternal copy of the gene).

Angelman syndrome can also be the result of mutation of a single gene. This gene (UBE3A, part of theubiquitinpathway) is present on both the maternal and paternal chromosomes, but differs in the pattern ofmethylation(imprinting). The paternal silencing of theUBE3Agene occurs in a brain region-specific manner; in thehippocampusandcerebellum, the maternal allele is almost exclusively the active one. The most common genetic defect leading to Angelman syndrome is an ~4Mb (mega base) maternal deletion in chromosomal region 15q11-13 causing an absence ofUBE3Aexpression in the paternally imprinted brain regions.UBE3Acodes for an E6-APubiquitin ligase, which chooses its substrates very selectively, and the four identified E6-AP substrates have shed little light on the possible molecular mechanisms underlying Angelman syndrome in humans.

Initial studies of mice that do not express maternalUBE3Ashow severe impairments in hippocampal memory formation. Most notably, there is a deficit in a learning paradigm that involves hippocampus-dependent contextualfear conditioning. In addition, maintenance of long-termsynaptic plasticityin hippocampal area CA1in vitrois disrupted inUbe3a-/-mice. These results provide links amongst hippocampalsynaptic plasticityin vitro, formation of hippocampus-dependent memoryin vivo, and the molecular pathology of Angelman syndrome.

Neurophysiology

One of the more notable features of Angelman Syndrome (AS) is the syndrome’spathognomonicneurophysiological findings. Theelectroencephalogram(EEG) in AS is usually very abnormal, and more abnormal than clinically expected. Three distinct interictal patterns are seen in these patients. The most common pattern is a very large amplitude 2–3Hz rhythm most prominent in prefrontal leads. Next most common is a symmetrical 4–6Hz high voltage rhythm. The third pattern, 3–6Hz activity punctuated by spikes and sharp waves in occipital leads, is associated with eye closure. Paroxysms of laughter have no relation to the EEG, ruling out this feature as agelasticphenomenon.

It appears that the neurons of patients with Angelman syndrome are formed correctly, but they cannot function properly.

Diagnosis

The diagnosis of Angelman syndrome is based on:

  • A history of delayed motor milestones and then later a delay in general development, especially of speech
  • Unusual movements including fine tremors, jerky limb movements, hand flapping and a wide-based, stiff-legged gait.
  • Characteristic facial appearance (but not in all cases).
  • A history of epilepsy and an abnormalEEGtracing.
  • A happy disposition with frequent laughter
  • A deletion or inactivity on chromosome 15 by array comparative genomic hybridization (aCGH) or by BACs-on-Beads technology.

Diagnostic criteria for the disorder were initially established in 1995 in collaboration with the Angelman syndrome Foundation (USA); these criteria have undergone revision in 2005

Treatment

There is currently no cure available. The epilepsy can be controlled by the use of one or more types of anticonvulsant medications. However, there are difficulties in ascertaining the levels and types of anticonvulsant medications needed to establish control, because AS is usually associated with having multiple varieties of seizures, rather than just the one as in normal cases of epilepsy. Many families use melatonin to promote sleep in a condition which often affects sleep patterns. Many individuals with Angelman syndrome sleep for a maximum of 5 hours at any one time. Mild laxatives are also used frequently to encourage regular bowel movements, and early intervention withphysiotherapyis important to encourage joint mobility and prevent stiffening of the joints.

Those with the syndrome are generally happy and contented people who like human contact and play. People with AS exhibit a profound desire for personal interaction with others. Communication can be difficult at first, but as a child with AS develops, there is a definite character and ability to make themselves understood. People with AS tend to develop strong non-verbal skills to compensate for their limited use of speech. It is widely accepted that their understanding of communication directed to them is much larger than their ability to return conversation. Most afflicted people will not develop more than 5–10 words, if any at all.

Seizures are a consequence, but so is excessive laughter,which is a major hindrance to early diagnosis.

Prognosis

The severity of the symptoms associated with Angelman syndrome varies significantly across the population of those affected. Some speech and a greater degree of self-care are possible among the least profoundly affected. Unfortunately, walking and the use of simple sign language may be beyond the reach of the more profoundly affected. Early and continued participation in physical, occupational (related to the development of fine-motor control skills), and communication (speech) therapies are believed to improve significantly the prognosis (in the areas of cognition and communication) of individuals affected by AS. Further, the specific genetic mechanism underlying the condition is thought to correlate to the general prognosis of the affected person. On one end of the spectrum, a mutation to theUBE3Ageneis thought to correlate to the least affected, whereas larger deletions on chromosome 15 are thought to correspond to the most affected.

The clinical features of Angelman syndrome alter with age. As adulthood approaches, hyperactivity and poor sleep patterns improve. The seizures decrease in frequency and often cease altogether and the EEG abnormalities are less obvious. Medication is typically advisable to those with seizure disorders. Often overlooked is the contribution of the poor sleep patterns to the frequency and/or severity of the seizures. Medication may be worthwhile to help deal with this issue and improve the prognosis with respect to seizures and sleep. Also noteworthy are the reports that the frequency and severity of seizures temporarily escalate in pubescent Angelman syndrome girls, but do not seem to affect long-term health.

The facial features remain recognizable with age, but many adults with AS look remarkably youthful for their age.

Pubertyandmenstruationbegin at around the average age.Sexual developmentis thought to be unaffected, as evidenced by a single reported case of a woman with Angelman syndrome conceiving a female child who also had Angelman syndrome.

The majority of those with AS achievecontinenceby day and some by night. Angelman syndrome is not a degenerative syndrome. Many people with AS improve their living skills with support.

Dressing skills are variable and usually limited to items of clothing without buttons or zippers. Most adults can eat with a knife or spoon and fork, and can learn to perform simple household tasks. General health is fairly good and life-span near average. Particular problems which have arisen in adults are a tendency to obesity (more in females), and worsening ofscoliosisif it is present. The affectionate nature which is also a positive aspect in the younger children may also persist into adult life where it can pose a problem socially, but this problem is not insurmountable.

History

"Boy with a Puppet" or "A child with a drawing" byGiovanni Francesco Caroto

Harry Angelman, a pediatrician working inWarrington, England, first reported three children with this condition in 1965. Angelman later described his choice of the title "Puppet Children" to describe these cases as being related to an oil painting he had seen while vacationing in Italy:

The history of medicine is full of interesting stories about the discovery of illnesses. The saga of Angelman's syndrome is one such story. It was purely by chance that nearly thirty years ago (e.g., circa 1964) three handicapped children were admitted at various times to my children's ward in England. They had a variety of disabilities and although at first sight they seemed to be suffering from different conditions I felt that there was a common cause for their illness. The diagnosis was purely a clinical one because in spite of technical investigations which today are more refined I was unable to establish scientific proof that the three children all had the same handicap. In view of this I hesitated to write about them in the medical journals. However, when on holiday in Italy I happened to see an oil painting in theCastelvecchio MuseuminVeronacalled . . . a Boy with a Puppet. The boy's laughing face and the fact that my patients exhibited jerky movements gave me the idea of writing an article about the three children with a title of Puppet Children. It was not a name that pleased all parents but it served as a means of combining the three little patients into a single group. Later the name was changed to Angelman syndrome. This article was published in 1965 and after some initial interest lay almost forgotten until the early eighties.

—Angelman quoted by Charles Williams

Case reports from the United States first began appearing in the medical literature in the early 1980s. In 1987, it was first noted that around half of the children with AS have a small piece ofchromosome 15missing (chromosome 15q partial deletion)

2-Von Hippel–Lindau(VHL) disease is arare,autosomal dominantgenetic conditionthat predisposes individuals to benign and malignant tumours. The most common tumours found in VHL arecentral nervous systemand retinalhemangioblastomas,clear cell renal carcinomas,pheochromocytomas, pancreaticneuroendocrine tumours, pancreatic cysts,endolymphatic sac tumorsand epididymal papillary cystadenomas.VHL results from amutationin thevon Hippel–Lindau tumor suppressorgene on chromosome 3p25.3.

Epidemiology

VHL disease has an incidence of one in 36,000 births. There is over 90%penetranceby the age of 65.

Signs and symptoms

Slit lamp photograph showingretinal detachmentin Von Hippel-Lindau disease.

Signs and symptoms associated with VHL disease include headaches, problems with balance and walking, dizziness, weakness of the limbs, vision problems, and high blood pressure. Conditions associated with VHL disease includeangiomatosis,hemangioblastomas,pheochromocytoma,renal cell carcinoma,pancreaticcysts(pancreatic serous cystadenoma),endolymphatic sac tumor, and bilateral papillary cystadenomas of theepididymis(men) orbroad ligament of the uterus(women). Angiomatosis occurs in 37.2% of patients presenting with VHL disease and usually occurs in the retina. As a result, loss of vision is very common. However, other organs can be affected: strokes, heart attacks, and cardiovascular disease are common additional symptoms. Approximately 40% of VHL disease presents with CNS hemangioblastomas and they are present in around 60-80%. Spinal hemangioblastomas are found in 13-59% of VHL disease and are specific because 80% are found in VHL disease. Although all of these tumours are common in VHL disease, around half of cases present with only one tumour type.

Genetics

The disease is caused by mutations of thevon Hippel–Lindau tumor suppressor(VHL) gene on the short arm ofchromosome 3(3p25-26). There are over 1500germline mutationsandsomatic mutationsfound in VHL disease.

Von Hippel–Lindau disease is inherited in anautosomal dominantpattern.

Every cell in the body has 2 copies of every gene. In VHL disease, one copy of the VHL gene has a mutation and produces a faulty VHL protein (pVHL). However, the second copy still produces a functional protein. Tumours form from only those cells where the second copy of the gene has been mutated. This is known as thetwo-hit hypothesis. A lack of this protein allows tumors characteristic of von Hippel–Lindau syndrome to develop

Approximately 20% of cases of VHL disease are found in individuals without a family history, known asde novomutations. An inherited mutation of the VHL gene is responsible for the remaining 80 percent of cases.

30-40% of mutations in the VHL gene consist of 50-250kbdeletion mutationsthat remove either part of the gene or the whole gene and flanking regions of DNA. The remaining 60-70% of VHL disease is caused by the truncation of pVHL bynonsense mutations,indel mutationsorsplice site mutations.

VHL disease subtypes

VHL disease can be subdivided according to the clinical manifestations, although these groups often correlate with certain types of mutations present in the VHL gene.

Type 1

Type one often hasdeletionornonsense mutations. This group manifests mostly as hemangioblastomas whereas clear-cell renal carcinomas and pheochromocytomas are rare.PELIK2 SAHAJA[

Type 2

Type 2 VHL disease is subdivided into Type 2A, B and C which are characterised mostly bymissense mutations. Type 2A is at risk of hemangioblastomas and pheochromocytomas, but not clear-cell renal carcinomas. Type 2B is at risk of all three tumours, with a higher risk of clear-cell renal carcinoma. Type 2C is at risk for only pheochromocytoma.

Type 3

Type 3 VHL disease has a risk of Chuvashpolycythaemia.

VHL protein